How not to generate a ±10 V power supply
I feel pretty dumb talking to nobody like this. My domain name doesn’t even work yet, but I guess logging is what engineers do. And I’m imaginative enough to see the utility of it: it’ll be nice down the line to have a log to review, writing stuff down helps flesh out ideas, and an open design process will make it a heck of a lot easier to produce open documentation. It will be tough to expose all my bad decisions and half-baked ignorance (and mixed metaphors), but I can suck it up.
My first project is to build an acoustic modem. This follows the principle of multiplying work: the modem doubles as my class project for ELEC 571: Underwater Acoustics. We’ve been using the Devantech SRF04 ultrasonic ranger in the mechatronics lab, and it strongly informs my design. The SRF04 actually uses an RS-232 chip to generate ±9 V levels, which actuate a piezoelectric transducer. I took a couple of the transducers from a broken SRF04 to use for my project.
I tried using an RS-232 chip that we had lying around in the lab, but those things have draconian current limits. There’s no way I can power a whole circuit off of one. I’m probably going to try it again soon though.
My solution: a DC/DC converter (AP34063N8L) to step up a 7.4 V lithium polymer battery up to 20 V. Taking half the output as the reference voltage will produce a ±10 V power supply. With that I can power pretty much anything I want.
I built the circuit shown above based on the AP34063N8L datasheet to step the battery up to 23 V. It then uses four 5 V regulators to generate 20 V, 15 V, 10 V, and 5 V rails, of which I take 10 V to be the reference. You may recognize that this circuit is completely ridiculous.
- Problem #1: the circuit, under no load, draws about 30 mA, or 0.25 W of power (loading it increases the power consumption as you’d expect)
- Problem #2: the 5 V regulators keep dying.
- Problem #3: I basically stuck capacitors and inductors anywhere that they wouldn’t cause problems.
Problem #1 is the major stumbling block. I will probably end up just using a battery pack or a few 9 V batteries to provide the ± rails. That’s way less cool than using a DC/DC converter (or even an RS-232 converter) but it’s looking like a pretty sweet idea right now.
Problem #2 is interesting. I’m not sure why, but I blew three regulators in one day. They weren’t generating detectable heat or anything. The circuit has the regulators chained together, so for example the one that generates 5 V is referenced to the negative rail and is powered by the regulator that generates 10 V. The 10 V rail is referenced to the 5 V regulator’s output, and is powered by the 15 V rail. And so on. I assume this is a horrible way to do it. It’s quite conceivable that the output pin on one of the middle regulators can’t sink current from a higher regulator, or maybe there’s some noise getting caught in a feedback loop. Anyway I bought some 10 V and 20 V regulators. I want to regulate the 20 V line to get rid of the ginormous ripple voltage coming from the step-up converter.
Problem #3 is mostly due to laziness: “I don’t feel like cutting another jumper wire, I’ll just use an inductor instead.” And more capacitors can’t hurt, right? Well, they also don’t help that much. I did buy some 15 mH inductors to try to filter the power supply a little better. They will be useless once I give up on this power supply and switch to 9 V batteries, but owning inductors makes me feel cool so it’s okay.
At this juncture it’s worth noting that I’m pretty much broke, so sometimes I’ll do things like build a crummy power supply because it’s cheaper (and niftier) than buying a few 9 V batteries, even though it’s actually way more expensive because I buy spare parts. This is not my most admirable quality, and hopefully keeping this public logbook will help me develop better habits through shame. On the bright side, my collection of useful components is expanding.